Global Affairs September 6, 2023 by

Raffaele Petroni

Prosecution of international crimes

Prosecution of international crimes: codification vs customary international law

The international community, over the decades following the second world conflict, has
extensively debated what would be the best way to prosecute international crimes. Both in
terms of debate about the development of legal institutions and what legal instruments and
sources would be more appropriate: whether codification or reliance on customary international

photo of gray concrete outdoor altar

The evolution of concepts and legal doctrines passed through the establishment of the International Military Tribunals (IMTs) of Nuremberg and Tokyo, universal jurisdiction and reliance on domestic courts, the International Criminal Tribunals for Former Yugoslavia and Rwanda (respectively ICTY and ICTR), the development of hybrid or internationalised courts and tribunals, and eventually the development of the International Criminal Court (ICC), which, although standing, sees its jurisdiction not recognised by all States.

The debate between codification and CIL focuses on whether codification is an undesirable development and if prosecution by CIL is a better option. Similarly, it is based on whether the benefits of codified international criminal law (ICL), which Van Schaak (2008) labels as a progressive approach, can offer better guarantees than the confidence that can be found in CIL for what concerns fair trials and the application of the principle of non crimen sine lege (NCSL).

The importance of this debate has also to do with how the prioritisation of competing legal
sources and principles is implemented.
The debate has brought to the analysis of the status system, particularly posing three questions:

  1. Did it happen? (conjecture);
  2. What has happened? (definition);
  3. What type of deed was it?
    (quality of the action). By answering these questions and how codification and CIL respond to
    these challenges each side has built its arguments.

One issue concerns the principle of NCSL: it is admitted that it is applicable both to CIL and
codified law with the same intensity. At the same time though, an argument against codification
is that criminal treaties are not negotiated by legal scholars, but by diplomats. This is considered
to be one of the reasons why the basic elements of crimes such as mens rea, actus reus, and
the existing circumstances are not included in the drafting and crimes remain vague in their
description. This argument is followed by the fact that treaties can be amended only through a
long, but infrequent, diplomatic process.

A controversial debate is the one that concerns positivism and the role of CIL: Van Schaak
(2008) argues that a complete and integrated positivistic approach is hardly to be implemented
and applicable where CIL is an essential part of the sources of ICL.

A controversial aspect of this debate is the application of the NCSL principle and its potential
flexibility: the breach of this principle in a conviction would be considered by the courts an
injustice, however, an even greater form of injustice would be considered the acquittal of a
perpetrator because of legal loopholes.

When dealing with CIL, the elements to consider concern the objective and purposes of ICL and
how these develop over time. An example of these developments have concerned the
applicability of international humanitarian law (IHL) in international and non-international
conflicts, particularly for what concerns superior responsibility. IHL, through additional protocol 2
(AP2), contains a variant and development of the Martens Clause (which contains an ensemble of uncodified law applicable in armed conflict in cases where treaty law does not completely
codify; it was set in 1899, when personal criminal responsibility had not been instituted yet)
concerning the principles of personal criminal responsibility even in cases where treaties are
silent on the matter and the one of command responsibility: through the ICTY, the principles of
command responsibility that were previously intended only for international conflicts now apply
to non international conflicts as well.

Other elements to consider are the effects that tribunals can have in the development of
deterrent principles. In this sense, ad hoc international tribunals, being ex post facto, do not
have an important impact. Other effects concern ICL being considered lex specialis: through this
doctrine, the law that focuses on special aspects overrides the law that focuses on general
aspects. Following this approach, courts and tribunals are considered able to update older law
to more actual realities.

For an act to be considered a violation of CIL it is necessary that the rule be customary or part
of an applicable convention: the breach must concern important values and encompass
individual criminal accountability. In these terms though, the absence of a criminal provision
from a convention does not imply that the perpetrator does not commit a crime.
One issue that came about during the IMT of Nuremberg concerned the applicability of
individual criminal responsibility on acts that were not considered crime when they were
committed (retroactive application). The violation of the principle of NCSL led to several lines of
thought in the court, among legal scholars, and among observers. The conclusion was that
convicting someone on an alleged violation of NCSL was preferable to acquitting the perpetrator
on this loophole as it would have represented a far deeper injustice. The court also established
that the principle was one of justice, not law, therefore it did not curb the ability of the winning
powers to prosecute based on those lines.

Another main consequence of the IMT Nuremberg is that illegality (in terms of state
) is equalised to criminality (individual criminal responsibility). While one of the
principles it applies is that of acts malum in se: through it, a link between immorality and
criminality is set. This allows to get over the NCSL for crimes that are immoral in se, intended as
acts that shock the conscience of the world. In these cases then, prosecution and punishment of
perpetrators is necessary.

The Norman case (Special Court of Sierra Leone, SCSL) is an example of this situation:
enlistment of child soldiers, although not widely and universally condemned, is condemnable as
intrinsically immoral.

Another principle that is relied on for determining the existence of law is that notice anywhere is
notice everywhere; this can happen through conventions and treaties, CIL – on the subject the
principle of nullum crimen sine jure is expected to be applicable. Depending on how CIL is
imported in the legal system, jure has to be intended not only as positive law. On the subject,
judge Robertson (UK) expressed the opinion that a tribunal must not check only whether a
certain behaviour was prohibited, but also whether the states intended to include personal
criminal responsibility (Van Schaak, 2008). His remarks concerned the Scilingo case: in that
situation, in his opinion, the case had to be assessed based on Spanish law applicable at the
time of the commission of the crime, not based on CIL. The line of thought that was followed
concerned the fact that CIL can fill the gaps existing in codified law, but not situations where the
domestic law does not expressly cover the matter.

Other elements that are used to determine the existence of law are the general principles of law,
judicial opinions, and the teachings of the qualified publicists. In these efforts, international
tribunals can rely also on domestic law as a source of existing notice. However, the difference
that exists in these cases is that the crimes described and formulated in domestic law do not
have the chapeau element that makes the crimes international: for example, for crimes against
humanity, the important characteristic is that they are committed in a systematic and
widespread manner.

In conclusion, the debate did not result in a final and clear position to be held generally.
Although CIL played an important role in the development of ICL, as time goes by, its relevance
is most likely to be declining leaving space for codification enterprises. On these terms, the
development of the ICC can be considered a type of development following this path. At the
same time though, the ICC statute itself, through art 21 (applicable law), reaffirms the role of CIL
and principles of law in filling the gaps. For these reasons, the debate is far from being solved.


Crimen Sine Lege: Judicial Lawmaking at the Intersection of Law and Morals
Van Shaack, 2008

About the author

Raffaele Petroni

Raffaele Petroni